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Date: Tue, 29 Mar 2022 21:02:47 -0600
Message-ID: <CAKWdMujkawYiV9dK-dEfGmnRfW_7ULu0n=3y6Wtn43y8fc66qQ@mail.gmail.com>
To: Guilliam Xavier <guilliam.xavier@gmail.com>
Cc: php internals <internals@lists.php.net>
Content-Type: multipart/alternative; boundary="0000000000006282e205db66c8b5"
Subject: Re: [PHP-DEV] Typed constants revisited
From: mbniebergall@gmail.com (Mark Niebergall)

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On Mon, Mar 28, 2022 at 6:54 AM Guilliam Xavier <guilliam.xavier@gmail.com>
wrote:

> Constants are not abstract in an interface - they must be assigned a
>> value. Only classes and methods can be abstract. Within an abstract class
>> it is not valid to have an abstract property. Properties can be defined
>> `protected int $id;` and optionally assigned a value `protected int $id =
>> 5;`, but cannot be `abstract protected int $id;`.
>>
>
> That's the *current* state of the language indeed, but to me, [part of]
> your proposal looks like introducing "abstract constants"... Maybe I'm
> misunderstanding?
>
>
>> So to me it makes more sense to have constants follow the same syntax as
>> properties `public const bool CAN_FLY;` without the `abstract` keyword.
>>
>> An example:
>>
>> ```
>> abstract class Bird
>> {
>>     public const bool CAN_FLY;
>>     protected bool $isExtinct;
>> ```
>>
>> This allows for similar behavior, similar requirements, and similar
>> syntax - consistency ftw!
>>
>
> For similarity/consistency, the `$isExtinct` property should probably be
> [`public` and] `static` (actually `readonly` too, but cannot be both).
>
> But, again, we can also see some similarity with `static` *methods*, e.g.:
>
> ```
> abstract class Bird
> {
>     abstract public const bool CAN_FLY;
>     abstract public static function isExtinct(): bool;
> }
> class Dove extends Bird
> {
>     // NOTE: the following two lines are required (in the class definition)
>     public const bool CAN_FLY = true;
>     public function static isExtinct(): bool { return false; }
> }
> // var_dump(Dove::CAN_FLY);
> // var_dump(Dove::isExtinct());
> ```
>
> Besides, an uninitialized property must be initialized before first read,
> but is *not* required to be overridden/redefined (with a value) in a child
> class; so in this example (still hypothetical):
>
> ```
> abstract class Bird
> {
>     public const bool CAN_FLY;
>     public static bool $isExtinct;
> }
> class Dodo extends Bird
> {
>     // NOTE: the following two lines are commented out
>     // public const bool CAN_FLY = false;
>     // public static bool $isExtinct = true;
> }
> var_dump(Dodo::CAN_FLY);
> var_dump(Dodo::$isExtinct);
> ```
>
> where would the [missing value for constant] error be: on the definition
> of class Dodo (like for an unimplemented abstract method), or only when
> trying to access Dodo::CAN_FLY (like for an uninitialized property)?
>
>
>>
>> There seems to be interest and good use cases (thanks Sara for the good
>> practical example!). At this point I'm going to work on a new RFC with all
>> the details and feedback from this discussion.
>>
>>
>
> Thanks & good luck! =)
>

I have updated the RFC https://wiki.php.net/rfc/typed_class_constants with
more details and examples from this thread, and updated the RFC status to
Under Discussion. Hopefully the updated RFC helps answer questions and
clarifies what the proposal includes.

Of note is the "Inheritance and variance" section, which details uses with
abstracts and interfaces, plus the "Constant values" section which includes
details about errors.


>
> --
> Guilliam Xavier
>

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